ABSTRACT: The predatory impacts of Cercopagis pengoi, a planktivorous cladoceran first observed in Lake Ontario in 1998, were assessed by examining patterns in the distribution and population dynamics of macrozooplankton. Specifically, we determined the vertical distribution of C. pengoi in Lake Ontario with respect to other zooplankton species and possible biotic and abiotic controlling factors and used seasonal time series of potential prey species, both before and after C. pengoi invasion, to make inferences about predatory impacts of C. pengoi. The highest abundances of C. pengoi generally occurred at the bottom of the epilimnion or at the top of the metalimnion, and strong diel vertical migration was not observed. The vertical distribution of cyclopoid and calanoid nauplii and copepodids was deeper in 1999 than had historically been observed, which suggest possible avoidance of C. pengoi or localized predatory depletion. Seasonal analyses of normalized biomass size spectra from the Bay of Quinte supported an impact of C. pengoi on organisms <0.1 µg dry weight. The hypothesized predatory effect on juvenile copepods was supported by an analysis of seasonal time series, where we observed that high abundances of C. pengoi coincided with marked decreases in these groups. Consequently, the presence of C. pengoi likely decreases the production of juvenile copepods both through direct predation and through a shift of copepod vertical distribution to colder waters. Among the cladocerans, only Bosmina longirostris abundance declined in the presence of C. pengoi in the analysis of seasonal time series.

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